The invention relates to a safety ascender for ascending on a rope, comprising a U-shaped clamp equipped with two lateral wings delineating an internal space for passage of the rope, and with an arresting surface with spikes against which the rope is blocked in case of a fall, the two wings being provided with two openings arranged facing one another and each having an edge in the form of an inclined ramp performing guiding of a karabiner passing through the two openings.